1. Field of the Invention
This invention relates to a motion detecting circuit for a digital video signal and, more particularly, to a motion detecting circuit used in a Y/C separator for separating a digital color video signal into the chrominance signal (C) and the luminance signal (Y).
2. Description of the Related Art
In order to improve image quality of a television picture, signal processing of a digital system has been recently performed in addition to a conventional analog processing upon detection of a video signal. Any existing composite color television signal is transmitted by multiplexing the luminance signal (to be referred to as the Y signal hereinafter) with the chrominance signals (to be referred to as the C signal hereinafter). For this reason, similar to the analog processing, a circuit for separating these two types of signal components is required in a digital processing section in a receiver side. Currently, a comb filter capable of separating signals without degrading a horizontal resolution is widely used as such a separating circuit. However, in such a comb filter, if a high-frequency component in a vertical direction is present because the C signal has low correlation in the vertical direction, this component may be mixed in the Y signal, thereby causing a color disturbance called dot interference on a monitor screen.
In the NTSC signal compared with the phase of the C signal of a current frame, that of the C signal of the immediately preceding frame is inverted. Therefore, there is provided a technique for the Y/C separator wherein the C signal can be obtained from the difference between the signals of the current frame and the immediately preceding frame, whereas the Y signal can be obtained from the sum of the signals. According to this technique, the C and Y signals can be obtained regardless of correlation of image in the vertical direction, without causing the above-described interference.
However, according to this technique, in a still image portion, the above two signals can be completely separated, whereas in a motion image portion, incomplete separation is caused.
In order to prevent such a trouble, a technique using a motion detecting type filter having two separating circuits has been developed. According to this technique, the motion adaptive type filter detects motion of the image and switches the separating circuits to be suitably used for the still and motion image portions, respectively.
In this technique, first motion data is obtained from the difference between digital video signals of adjacent frames, and second motion data is obtained from the difference between digital video signals of every other frame. The first and second data are used for obtaining a motion detection signal varying in accordance with the motion of a digital video signal (image). By performing Y/C separation using this motion detection signal obtained in this manner, complete Y/C separation can be performed for the motion image portion as well as the still image portion.
However, in order to obtain the second motion data, the motion detecting circuit according to the above idea requires two frame memories, each having a memory capacity of one frame, i.e., a large memory having a capacity of two frames, as a whole. Therefore, it costs a great deal to realize the motion detecting circuit according to this idea. In addition, a large quantity of hardware is required to realize an IC as a digital TV system by utilizing this motion detecting circuit.